Steroid hormones were measured in fetal serum at the time of gonadal differentiation (days 38 to 40 of gestation). Male fetuses had significantly more testosterone in their circulatory systems than female fetuses. Other androgens such as androstenedione or dihydrotestosterone did not differ with the sex of the fetus. Although these studies have not demonstrated when the fetal tested begin the secrete testosterone, they have demonstrated the presence of the hormone during a sensitive period of male development. The fetal ovaries produce estrogen early in gestation if provided with the proper stimuli (cyclic AMP) in vitro. Control mechanisms for gonadal function in the fetus are under investigation.